Oxidative Alteration of Uraninite at the Nopal I Deposit, Mexico: Possible Contaminant Transport and Source Term Constra
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OXIDATIVE ALTERATION OF URANINITE AT THE NOPAL I DEPOSIT, MEXICO: POSSIBLE CONTAMINANT TRANSPORT AND SOURCE TERM CONSTRAINTS FOR THE PROPOSED REPOSITORY AT YUCCA MOUNTAIN BRET W. LESLIE, ENGLISH C. PEARCY, AND JAMES D. PRIKRYL
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228-0510 ABSTRACT
The Nopal I uranium deposit at Pefla Blanca, Mexico is being studied as a natural analog of the proposed high-level nuclear waste repository at Yucca Mountain. Identification of secondary uranium phases at Nopal I, and the sequence of their formation after uraninite oxidation, provides insight into the source term for uranium, and suggests that uranophane may control uranium release and transport in a silicic, tuffaceous, chemically oxidizing, and hydrologically unsaturated environment. Possible constraints on contaminant transport at Nopal I are derived from the spatial distribution of uranium and from measurements of 238 U decay-series isotopes. The analyses indicate that flow of U-bearing fluids was influenced strongly by fracture density, but that the flow of these fluids was not restricted to fractures. Gamma spectroscopic measurements of 238U decayseries isotopes indicate secular equilibrium, which suggests undetectable U transport under present conditions. INTRODUCTION
The Nopal I deposit has been the focus of earlier research [1-4], and these studies provide considerable background on the mineralogy and alteration at Nopal I. Analogous aspects of the Nopal I deposit to the proposed repository at Yucca Mountain include the silicic, tuffaceous host rocks, the semi-arid climate, the oxidizing chemical environment, the unsaturated hydrologic regime, and the presence of reduced uranium mineralization [5,6]. An interpretation of the evolution of the deposit [5,7] and some preliminary source-term constraints [5,8] have been derived. The Nopal I uranium deposit is hosted by rhyolitic tuffs of the Nopal and Coloradas Formations [5], and the uranium mineralization is located within a "breccia pipe structure" (20 x 40 m wide, 100 m high) [2]. Exposure of the ore body at Nopal I is unusually good, with the mineralization presently exposed on broad excavated benches at the 0 and + 10 m levels (Figure 1). Clearing of a portion of the + 10 m level and the emplacement of a permanent sampling grid (Figure 2) has resulted in an extraordinary exposure of the deposit and the opportunity to determine the geologic and mineralogic relationships which comprise the analog [9]. MINERALOGY AND PARAGENESIS
Primary uranium mineralization at Nopal I consists of a uraninite-kaolinite-quartz-pyrite assemblage [2,5] that is only preserved as millimeter to centimeter-sized masses within strongly silicified breccia within the ore "pipe" in the level 0 m adit [5,9]. Alteration of the primary assemblage has led to (i) the oxidation of uraninite and the formation of a suite of microcrystalline and intimately intergrown uranyl oxide hydrate and uranyl silicate minerals, and (ii) the oxidati
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